The Professors Imanishi and Wengel independently invented Locked Nucleic Acid (LNA) in 1997 (International Patent Applications WO 99/14226, WO 98/39352; P. Nielsen et al, J. Chem. Soc., Perkin Trans. 1, 1997, 3423; P. Nielsen et al., Chem. Commun., 1997, 9, 825; N. K. Christensen et al., J. Am. Chem. Soc., 1998, 120, 5458; A. A. Koshkin et al., J. Org. Chem., 1998, 63, 2778; A. A Koshkin et al. J. Am. Chem. Soc. 1998, 120, 13252-53; Kumar et al. Bioorg, & Med. Chem. Lett., 1998, 8, 2219-2222; and S. Obika et al., Bioorg. Med. Chem. Lett., 1999, 515). The first LNA monomer was based on the 2′-O—CH2-4′ bicyclic structure. Due to the configuration of this structure it is called: beta-D-oxy-LNA. This oxy-LNA has since then showed promising biological applications (Braasch & Corey, Biochemistry, 2002, 41(14), 4503-19; Childs et al. PNAS, 2002, 99(17), 11091-96; Crinelli et al., Nucl. Acid. Res., 2002, 30(11), 2435-43; Elayadi et al., Biochemistry, 2002, 41, 9973-9981; Jacobsen et al., Nucl. Acid. Res., 2002, 30(19), in press; Kurreck et al., Nucl. Acid. Res., 2002, 30(9), 1911-1918; Simeonov & Nikiforov, Nucl. Acid. Res., 2002, 30(17); Alayadi & Corey, Curr. opinion in Inves. Drugs., 2001, 2(4), 558-61; Obika et al., Bioorg. & Med. Chem., 2001, 9, 1001-11; Braasch & Corey, Chem. & Biol., 2000, 55, 1-7; Wahlestedt et al., PNAS, 2000, 97(10), 5633-38), Freier & Altmann, Nucl. Acid Res., 1997, 25, 4429-43; Cook, 1999, Nucleosides & Nucleotides, 18(6&7), 1141-62.
Right after the discovery of oxy-LNA the bicyclic furanosidic structure was chemically derivatised. Thus, the 2′-S—CH2-4′ (thio-LNA) and the 2′—NH—CH2-4′ (amino-LNA) bicyclic analogues were disclosed (Singh, S. K., J. Org. Chem., 1998, 63, 6078-79; Kumar et al. Bioorg, & Med. Chem. Lett., 1998, 8, 2219-2222; Singh et al. J. Org. Chem., 1998, 63, 10035-39). The synthesis of the thio-LNA containing uridine as nucleobase has been shown (Singh, S. K., J. Org. Chem., 1998, 63, 6078-79). For amino-LNA the synthesis of the thymidine nucleobase has been disclosed (Kumar et al. Bioorg, & Med. Chem. Lett., 1998, 8, 2219-2222; Singh et al. J. Org. Chem., 1998, 63, 10035-39). A series of LNA-diastereoisomers have been prepared (Rajwanshi et al., J. Chem. Commun. 1999; 2073-2074; Hakansson & Wengel, Bioorg Med Chem Lett 2001; 11(7):935-938; Rajwanshi et al., Chem. Commun., 1999; 1395-1396; Wengel at al., Nucleosides Nucleotides Nucleic Acids, 2001; 20(4-7):389-396; Rajwanshi et al., Angew. Chem. Int. Ed., 2000; 39:1656-1659; Petersen et al., J. Amer. Chem. Soc., 2001, 123(30), 7431-32; Sørensen et al., J. Amer. Chem. Soc., 2002, 124(10), 2164-76; Vester et al., J. Amer. Chem. Soc., 2002, 124(46), 13682-13683). In the prior art the synthesis of alpha-L-xylo, xylo-LNA, and alpha-L-oxy-LNA containing thymidine bases have been shown. For the alpha-L-oxy-LNA also the 5-methyl and adenine nucleosides have been synthesised. The melting temperature (Tm) of duplexes containing the LNA distereoisomers have been presented. It turned out that the alpha-L-oxy-LNA has interesting properties. It was shown that the alpha-L-oxy-LNA can be incorporated in complex chimerae comprising DNA/RNA residues and be adapted in the oligo structure and increase the binding. This property of being incorporated in oligonucleotides containing several other monomeric classes and act co-operatively is a property that the alpha-L-oxy-LNA shares with the parent oxy-LNA. Furthermore, it has been demonstrated that a segment of 4 consecutive alpha-L-T monomers can be incorporated in conjunction with a segment of 4 consecutive oxy-LNA-T monomers (Rajwanshi et al., Chem. Commun., 1999, 2073-74). Increased stability of oligonucleotides containing alpha-L-oxy-LNA monomers (MeC, A, T-monomers) have been demonstrated. The alpha-L-oxy-LNA monomers were incorporated into oligonucleotides with alternating alpha-L-oxy-LNA and DNA monomers (mix-mers) and in fully modified alpha-L-oxy-LNA oligomers. The stability was compared to oxy-LNA and to DNA and it was found that alpha-L-oxy-LNA monomers displaced the same protection pattern as oxy-LNA (Sørensen, et al., J. Amer. Chem. Soc., 2002, 124(10), 2164-76). The same alpha-L-oxy-LNA containing oligonucleotides were tested in RNase H assays and it was found that the designs disclosed were not efficiently recruiting RNase H. When these examples are taken together also in combination with the data published by Arzumanov et al (Biochemistry 2001, 40, 14645-54) it has not been shown that alpha-L-oxy-LNA containing oligonucleotides efficiently recruits RNase H.
Oligonucleotides containing any combination of the diastereoisomers and any other LNA family member has not been demonstrated.
Natural dsDNA exists at physiological pH as a B-form helix, whereas dsRNA exists as an A-form helix. A helix formed by DNA and RNA exists in an intermediate A/B-form. This morphological difference is originated in the difference in the preferred sugar conformations of the deoxyriboses and the riboses. The furanose ring of deoxyribose exists at room temperature in an equilibrium between C2′-endo (S-type) and C3′-endo (N-type) conformation with an energy barrier of ˜2 kcal/mol (FIG. 3). For deoxyribose the S-type conformation is slightly lowered in energy (˜0.6 kcal/mol) compared to the N-type and explains why DNA is found in the S-type conformation. The conformation leads to the B-form helix. For ribose, and RNA, the preference is for the N-type that leads to the A-form helix. The A-form helix is associated with higher hybridisation stability. The oxy-LNA and the LNA analogues are locked in the N-conformation and consequently the oligonucleotides they are forming will be RNA-like. The alpha-L-oxy-LNA is locked in a S-type and therefore the oligonucleotides that it will form will be more DNA like (Sørensen et al., J. Amer. Chem. Soc., 2002, 124(10), 2164-76; Rajwanshi et al., Angew. Chem. Int. Ed., 2000; 39:1656-1659). Molecular strategies are being developed to modulate unwanted gene expression that either directly causes, participates in, or aggravates a disease state. One such strategy involves inhibiting gene expression with oligonucleotides complementary in sequence to the messenger RNA of a target gene. The messenger RNA strand is a copy of the coding DNA strand and is therefore, as the DNA strand, called the sense strand. Oligonucleotides that hybridise to the sense strand are called antisense oligonucleotides. Binding of these strands to mRNA interferes with the translation process and consequently with gene expression. Zamecnik and co-workers originally described the Antisense strategy and the principle has since then attracted a lot of interest (Zamecnik & Stephenson, PNAS, 1978, 75(1), 280-4; Bennet & Cowset, Biochim. Biophys. Acta, 1999, 1489, 19-30; Crooke, 1998, Biotechnol. Genet. Eng Rev., 15, 121-57; Wengel, J. In Antisense Drug Technology; Principles, Strategies, and Applications; Edited by Crooke, S. T., Ed.; Marcel Dekker, Inc.: New York, Basel, 2001; pp 339-357).
It has been a long sought goal to develop drugs with the capacity to destroy malignant genes base specifically. The applications of such drugs in e.g. cancer and infections diseases are self-evident. Native oligonucleotides cannot be employed as such mainly due to their instability in cellular media and to too low affinity for the target genes. The wish to develop nucleic acid probes with improved properties in this regard has been the main driver behind the massive synthesis effort in the area of nucleic acid analogue preparation. The most important guideline in this work has been to design the DNA analogues in such a way that the DNA analogue would attain the N-type/“RNA”-like conformation that is associated with the higher affinity of the oligonucleotides to nucleic acids.
One of the important mechanisms involved in Antisense is the RNase H mechanism. RNase H is an intra cellular enzyme that cleaves the RNA strand in RNA/DNA duplexes. Therefore, in the search for efficient Antisense oligonucleotides, it has been an important hallmark to prepare oligonucleotides that can activate RNase H. However, the prerequisite for an oligonucleotide in this regard is therefore that the oligo is DNA-like and as stated above most high affinity DNA analogues induces RNA-like oligonucleotides. Therefore, to compensate for the lack of RNase H substrate ability of most DNA analogues (like e.g. 2′-OMe DNA analogue and oxy-LNA) the oligonucleotides must have segments/consecutive stretches of DNA and/or phosphorothioates. Depending on the design of the segments of such oligonucleotides they are usually called Gap-mers, if the DNA segment is flanked by the segments of the DNA analogue, Head-mers, if the segment of the DNA analogue is located in the 5′ region of the oligonucleotide, and Tail-mers, if the segment of the DNA analogue is located in the 3′ region of the oligonucleotide.
It should be mentioned that other important mechanisms are involved in Antisense that are not dependent on RNase H activation. For such oligonucleotides the DNA analogues, like LNA, can be placed in any combination design (Childs et al. PNAS, 2002, 99(17), 11091-96; Crinelli et al., Nucl. Acid. Res., 2002, 30(11), 2435-43; Elayadi et al., Biochemistry, 2002, 1, 9973-9981; Kurreck et al., Nucl. Acid. Res., 2002, 30(9), 1911-1918; Alayadi & Corey, Curr. opinion in Inves. Drugs., 2001, 2(4), 558-61; Braasch & Corey, Chem. & Biol., 2000, 55, 1-7).
In contrast to the beta-D-oxy-LNA the alpha-L-oxy-LNA has a DNA-like locked conformation and it has been demonstrated that alpha-L-oxy-LNA can activate RNase H (Sørensen et al., J. Amer. Chem. Soc., 2002, 124(10), 2164-76). However, the cleavage rate of RNase H is much lower compared to DNA in the disclosed designs and thus, the oligonucleotides in the disclosed designs have not been shown to be efficient Antisense reagents.